Gas is present in pulp suspensions mainly in three forms, namely, in the form of small bubbles, dissolved or chemical bound gas.
The chemically bound gas or dissolved gas seldom causes problems in the pulp and papermaking processes but can cause problems if conditions are changed and bubbles start to form.
Gas bubbles in the fiber suspension can be present as free bubbles in the liquid between the fibers or as bound bubbles attached to fibers. Both bound and free bubbles cause problems in the papermaking processes. Free bubbles cause special problems in the pulp and papermaking processes when they are present in too great an amount. The problems include foam problems, instability of the processes, decreased deaerating, and the like.
The method of the present invention relates to the separation and removal of most of the free air bubbles so that the problems caused by an excess amount of free air bubbles is eliminated.
Total gas removal is generally accomplished by another type of gas removal, so-called mechanical gas separation. With this method, all of the free and bound gas bubbles are removed. Also part of the dissolved gas is removed. This type of gas removal is performed immediately in front of the paper machine forming section to avoid pinholes and other problems on the forming wire. This method, which is described by K. D. Kurz, Tappi Engineering Conference, Sept. 19-21, (1978), is expensive and creates large amounts of foam when the fiber suspension is ejected with high speed onto a metal surface in a vacuum tank.
The traditional degassing assemblies in the pulp and paper industry are remarkably space demanding and hence costly, and the separated gas occurs in large volumes, from which reclaiming and conveying thereof is difficult. The most usual degassing equipment is a tank having a large diameter in which the gas in a gas contained liquid is permitted to rise to the liquid surface of the tank for removal. In order to be certain that a sufficient time period for degassing is given, the diameter of such a tank in large pulp plants can be 10-20 m and the height 5-6 m. It will thus be apparent to persons of ordinary skill that investment costs for a degassing tank of this kind are high and the reclaiming of gas therefrom is difficult.
According to a prior art technique differences in surface characteristics, rather than density, can be utilized to separate many materials by froth flotation processes. These material suspensions, be they coal, ores, fiber suspensions in the pulp and paper industry, or the like, are hereinafter referred to as suspensions. Flotation washing systems generally include one or more special flotation cell or tank which is an open or closed vat, or a series of vats, so as to provide a large free liquid surface for the formation of air bubbles. Each flotation cell is generally equipped with a high-speed agitator and an overflow for froth removal. The agitator causes gas to enter the system, to combine with the paper stock and flotation chemicals and to form air bubbles, e.g. in flotation deinking processes ink-laden air bubbles which will rise to the surface of the cell to be removed therefrom. As will be further discussed below, the separation operation can be improved by the addition of air into the flotation cell or by feeding into the flotation cell of soap-type chemicals such as fatty acid soaps with a suitable feed pump.
A partition wall or overflow is generally provided inside the flotation cell which overflow extends to a predetermined height thereby defining an upper limit for the liquid surface formed by the suspension inside the cell. The suspension inside the cell is treated with air and chemicals for generating upward rising air bubbles. Ink particles, pigments or other contaminants attached to the surface of the air bubbles while they rise to the surface of the suspension to form foam or froth thereon. The foam or froth together with the particles stuck thereto will thereafter flow over the partition wall mentioned above or may be skimmed off with a mechanical paddle or, e.g. by directing the foamy surface layer into a separate outlet duct for removing the contaminant containing fraction for disposal or further recovery, if so desired. The treated suspension, so-called floated suspension, is thereafter transferred from the bottom section of the flotation cell for further processing.
However, this prior art technique will only achieve a partial separation because not all of the material or all of the particles intended to be separated will be sufficiently attracted to the rising air bubbles, rather, part of the particles as well as air will remain in the suspension. Particles remaining in the suspension, if they are valuable are hard to be recovered or entirely lost, or, if the particles are of no value, the continued presence thereof will invariably cause problems in most subsequent process steps. Problems caused by the presence of gas in the suspension include pump cavitation and, especially, in the pulp industry the presence of air causes the following problems: